Newly discovered ‘ghost galaxy’ full of dark matter is as big as the Milky Way

Astronomers have discovered, with the aid of some powerful telescopes, a galaxy made up mostly of dark matter. Dragonfly 44, as it has been named, is roughly the same size as our Milky Way but with far fewer stars. Rather, the galaxy appears to be composed largely of dark matter, which does not emit light or interact with electromagnetic radiation. Although there is much more to learn about the mysterious dark galaxy, scientists’ initial findings have surprised astronomy experts more than once.

Studies of Dragonfly 44 began with curiosity, as many deep space explorations do, after it was identified last year as little more than a smudge-like spot on an image of the Coma Cluster of galaxies captured by the Sloan Digital Sky Survey (above left). Right away, astronomers knew they had to investigate, and time-lapse images captured by the Gemini North telescope (above right) show the galaxy’s diffuse nature. “Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together,” Yale University astronomer Pieter van Dokkum, lead author of the study, told Phys.org.

More powerful equipment was needed to get a better look, so the team turned to the W.M. Keck Observatory and the Gemini North telescope, in Hawaii, for help. Van Dokkum’s team was able to measure the velocity of stars in Dragonfly 44 by comparing images taken over six nights. Star velocity is a key element in gaining an understanding of the composition of a far-away galaxy, because it can help convey the galaxy’s mass. A higher velocity suggests a galaxy of higher mass.

Knowing that the galaxy had very few stars (and thus not much light) but a mass closer to that of the Milky Way, researchers concluded that the newly discovered galaxy must be comprised mostly of dark matter. “Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass,” said co-author Roberto Abraham of the University of Toronto. Further studies of Dragonfly 44 may help scientists finally come to an understanding of what dark matter actually is, which has eluded researchers since its existence was first suggested nearly a century ago.

A paper on the initial study of Dragonfly 44 was recently published in the Astrophysical Journal Letters.